The present invention relates to a combustion method for NOx reduction, as well as an apparatus therefor, to be applied to water-tube boilers, reheaters of absorption refrigerators, or the like.
Generally, as the principle of suppression of NOx generation, there have been known (1) suppressing the temperature of flame (combustion gas), (2) reduction of residence time of high-temperature combustion gas, and (3) lowering the oxygen partial pressure. Then, various NOx reduction techniques to which these principles are applied are available. Examples that have been proposed and developed into practical use include the two-stage combustion method, the thick and thin fuel combustion method, the exhaust gas recirculate combustion method, the water addition combustion method, the steam jet combustion method, the flame cooling combustion method with water-tube groups, and the like.
With the progress of times, NOx generation sources even of relatively small capacity such as water-tube boilers have been coming under increasingly stricter regulation of exhaust gas, and so further reduction of NOx is demanded therefor. The present applicant proposed NOx reduction techniques for these demands by the Specification of U.S. Pat. No. 6,029,614 and the like.
However, the extent of NOx reduction by these prior arts is up to about 25 ppm actually, and there has not yet been developed so far any NOx reduction technique of under 10 ppm in practical use. It is noted that NOx reduction with the value of NOx generation being not more than 10 ppm will hereinafter be referred to as super NOx reduction.
The reason of that lies in that NOx reduction and CO reduction are contradictory technical issues. That is, if the combustion gas temperature is abruptly lowered to facilitate NOx reduction so that the temperature is suppressed to as low as 900xc2x0 C. or less, a large amount of CO is generated and moreover the generated CO is discharged as it is unoxidized, with a result of increased CO emission. Conversely, if the combustion gas temperature is suppressed to a rather higher one in order to reduce the CO emission, the suppression of NOx generation becomes insufficient.
The NOx reduction technique proposed in the aforementioned prior art is also intended to suppress the combustion gas temperature so that the quantity of CO generated along with the NOx reduction is minimized, and that generated CO is oxidized. As a result of this, it has been the case that the prior art technique is limited in the selection of means for NOx reduction and poor in the suppression of combustion gas temperature, hence incapable of fulfilling the super NOx reduction.
An object of the present invention is to provide a combustion method for NOx reduction, as well as an apparatus therefor, capable of facilitating NOx reduction without requiring considerations of CO generation and easily achieving NOx reduction with the value of exhaust NOx under 10 ppm, and still capable of achieving CO reduction at the same time.
The present invention having been accomplished to solve the above object, in a first aspect of the invention, there is provided a NOx reduction combustion method for fulfilling NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: a NOx reduction step for suppressing combustion gas temperature in such a manner that suppression of NOx generation is preferred to reduction of exhaust CO value, thereby keeping NOx value not more than a specified value; and a CO reduction step for thereafter reducing exhaust CO value resulting from the NOx reduction step to not more than a specified value.
In a second aspect of the invention, there is provided a NOx reduction combustion method for fulfilling NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: a NOx reduction step for suppressing combustion gas temperature in such a manner that suppression of NOx generation is preferred to reduction of exhaust CO value, thereby keeping NOx value not more than 10 ppm (at 0% O2 in the exhaust gas, dry basis); and a CO reduction step for thereafter reducing exhaust CO value resulting from the NOx reduction step to not more than a specified value.
In a third aspect of the invention, there is provided a NOx reduction combustion method for fulfilling NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: a NOx reduction step for suppressing combustion gas temperature in such a manner that suppression of NOx generation is preferred to reduction of exhaust CO value, thereby keeping NOx value not more than a specified value; and a CO reduction step for thereafter reducing exhaust CO value resulting from the NOx reduction step to not more than a specified value, the CO reduction step being performed in a zone where the combustion gas temperature is not more than 900xc2x0 C.
In one embodiment, there is provided a NOx reduction combustion method as described in any one of the first to third aspects, wherein the NOx reduction step is performed with an excess air ratio which is determined from a NOx reduction target value and an excess air ratio versus NOx characteristic of the NOx reduction step.
In one embodiment, there is provided a NOx reduction combustion method as described in any one of the first to third aspects, wherein the CO reduction step is performed with a CO oxidation catalyst member.
In a fourth aspect of the invention, there is provided a NOx reduction combustion apparatus for fulfilling NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: NOx reduction means for suppressing combustion gas temperature in such a manner that suppression of NOx generation is preferred to reduction of exhaust CO value, thereby keeping NOx value not more than a specified value; and CO reduction means for reducing exhaust CO value resulting from the NOx reduction means to not more than a specified value.
In a fifth aspect of the invention, there is provided a NOx reduction combustion apparatus for fulfilling NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: NOx reduction means for suppressing combustion gas temperature in such a manner that suppression of NOx generation is preferred to reduction of exhaust CO value, thereby keeping NOx value not more than 10 ppm (at 0% O2 in the exhaust gas, dry basis); and CO reduction means for reducing exhaust CO value resulting from the NOx reduction means to not more than a specified value.
In a sixth aspect of the invention, there is provided a NOx reduction combustion apparatus for fulfilling NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: NOx reduction means for suppressing combustion gas temperature in such a manner that suppression of NOx generation is preferred to reduction of exhaust CO value, thereby keeping NOx value not more than a specified value; and CO reduction means for reducing exhaust CO value resulting from the NOx reduction means to not more than a specified value in a zone where the combustion gas temperature is not more than 900xc2x0 C.
In one embodiment, there is provided a NOx reduction combustion apparatus as described in any one of the fourth to sixth aspects, wherein the NOx reduction is performed with an excess air ratio which is determined from a NOx reduction target value and an excess air ratio versus NOx characteristic (NOx emission characteristic) of the NOx reduction means.
In one embodiment, there is provided a NOx reduction combustion apparatus as described in any one of the fourth to sixth aspects, wherein the CO reduction means is a CO oxidation catalyst member.
In one embodiment, there is provided a NOx reduction combustion apparatus as described in any one of the fourth to sixth aspects, wherein the NOx reduction means is implemented by heat transfer tubes having a space formed by removing heat transfer tubes.
Furthermore, in one embodiment, there is provided a NOx reduction combustion apparatus as described in any one of the fourth to sixth aspects, wherein the NOx reduction means is implemented by heat transfer tubes having no space formed by removing heat transfer tubes.
Further, aspects of the present invention will be described according to the embodiments. Before the description of embodiments, terms used herein and the drawings are explained. The combustion gas includes burning-reaction ongoing (under-combustion-process) combustion gas, and combustion gas that has completed burning reaction. Then, the burning-reaction ongoing gas refers to combustion gas that is under burning reaction, and the burning-completed gas refers to combustion gas that has completed burning reaction. The burning-reaction ongoing gas is indeed a concept of substance, but can also be referred to as flame as a concept of state because it generally includes a visible flame so as to be in a flame state. Therefore, herein, the burning-reaction ongoing gas is referred to also as flame or burning flame from time to time. Further, the exhaust gas (flue gas) refers to burning-completed gas that has decreased in temperature under an effect of endothermic action by heat transfer tubes or the like.
Also, the combustion gas temperature, unless otherwise specified, means the temperature of burning-reaction ongoing gas, equivalent to combustion temperature or combustion flame temperature. Further, the suppression of combustion gas temperature refers to suppressing the maximum value of combustion gas (combustion flame) temperature to a low one. In addition, normally, burning reaction is continuing although in a trace amount even in the burning-completed gas, and so the combustion completion does not mean a 100% completion of burning reaction.
Further, the excess air ratio, which is the actual amount of combustion air/theoretical amount of combustion air, corresponds in a specified relationship to exhaust-gas O2(%) (oxygen concentration in exhaust gas), therefore being expressed in exhaust-gas O2(%). Also, the value of NOx shows a value at 0% O2 in the exhaust gas, dry basis, while the value of CO shows not an equivalent value but a reading value.
Next, as a detailed description of the foregoing characteristics of the present invention, embodiments of the present invention are described. The present invention is applied to thermal equipment (or combustion equipment) such as small-size once-through boilers or other water-tube boilers, water heaters, reheaters of absorption refrigerators or the like. The thermal equipment has a burner and a group of heat absorbers to be heated by combustion gas derived from the burner.
An embodiment of the method according to the present invention is a NOx reduction combustion method for fulfilling NOx reduction by suppressing temperature of combustion gas jetted out from a burner, comprising: a NOx reduction step for suppressing combustion gas temperature in such a manner that suppression of NOx generation is preferred to reduction of exhaust CO value, thereby keeping the value of generated NOx not more than a specified value; and a CO reduction step for thereafter reducing exhaust CO value resulting from the NOx reduction step to not more than a specified value. This NOx reduction and CO reduction combustion method has been achieved by focusing on the characteristic that NOx, once generated, will hardly disappear while CO can be easily reduced after its generation, the method being a novel, useful combustion method and NOx-reduction and CO-reduction method in which the NOx reduction step is first preferentially performed so that the generated NOx value becomes a reduction target NOx value, and subsequently, the CO reduction step is performed.
First, in the NOx reduction step, the combustion gas temperature is suppressed by the NOx reduction means, so that the generated NOx value is reduced to not more than a specified value. The specified value is not more than a NOx value that has been achieved so far, and preferably not more than 10 ppm. In this NOx reduction step, NOx reduction is carried on in preference to the reduction of exhaust CO value, i.e., suppression of CO generation and acceleration of CO oxidation. This preference refers to the suppression of combustion gas temperature as much as possible under the condition of continuity of combustion, that is, the first execution of NOx reduction prior to CO reduction, and then the execution of the CO reduction subsequent to the NOx reduction, and also refers to the carrying-on of NOx reduction with CO reduction sacrificed or neglected out of NOx reduction and CO reduction, which are contradictory technical issues.
This NOx reduction step is explained in more detail. The NOx reduction step has an excess air ratio versus NOx characteristic that the generated NOx value decreases with increasing excess air ratio of the burner, as well as an excess air ratio versus CO characteristic that the exhaust CO value increases with increasing excess air ratio. In the NOx reduction step, an excess air ratio that causes the NOx value to become not more than a NOx reduction target value is determined under the condition of the excess air ratio versus NOx characteristic of this step, and the burner is burned at the resulting excess air ratio to do the NOx reduction. For the determination of this excess air ratio, the excess air ratio versus CO characteristic of the NOx reduction step is not taken into consideration.
Subsequently, in the CO reduction step, the value of CO generated and exhausted in the NOx reduction step is reduced to not more than a specified value by the CO reduction means. The specified value for this exhaust CO is 50 ppm, preferably, 20 to 30 ppm.
In this way, both a NOx reduction for the exhaust NOx value of not more than 10 ppm and a CO reduction for the exhaust CO value of not more than 50 ppm can be fulfilled.
Next, the NOx reduction step and the CO reduction step are described in terms of constitution.
The NOx reduction step includes various modes. A preferable mode thereof is that the step is carried out by NOx reduction means which comprises in combination: a combustion-gas-temperature suppression means for doing the suppression by burning a fully-premixing type gas burner at a high excess air ratio (hereinafter, referred to as xe2x80x9cfirst suppression meansxe2x80x9d); a combustion-gas-temperature suppression means for doing the suppression by heat absorbers (hereinafter, referred to as xe2x80x9csecond suppression meansxe2x80x9d); a combustion-gas-temperature suppression means for doing the suppression by recirculating burning-completed gas to a burning reaction zone (hereinafter, referred to as xe2x80x9cthird suppression meansxe2x80x9d); and a combustion-gas-temperature suppression means for doing the suppression by addition of water or addition of steam (hereinafter, referred to as xe2x80x9cwater/steam additionxe2x80x9d) to the burning reaction zone (hereinafter, referred to as xe2x80x9cfourth suppression meansxe2x80x9d). The burning reaction zone refers to a zone where burning-reaction ongoing gas is present.
The first suppression means is based on the following principle. That is, when the burner is burned at a high excess air ratio, the combustion gas temperature is suppressed so that the NOx value decreases. The high excess air ratio in this case is 5% O2 or more contained in exhaust gas, preferably, not less than 5.5% O2. This suppression effect acts generally uniformly on the entire burning reaction zone formed by the burner.
The second suppression means is based on the following principle. That is, the NOx value is reduced by suppressing the combustion gas temperature by a cooling effect of heat absorbers implemented by arranging a multiplicity of heat absorbers in the burning-reaction ongoing gas derived from the burner, i.e., in the burning reaction zone. This second suppression means is implemented by arranging the heat absorbers to cool the burning-reaction ongoing gas, hence a nonuniform cooling. There are also sites where the burning is ongoing actively in the gaps between the heat absorbers of the burning reaction zone. Particularly in the downstream of the heat absorbers, eddy currents are formed so that the combustion flame is stabilized by the heat transfer tubes. The heat absorbers are implemented by heat transfer tubes such as water tubes, but this is not limitative.
The arrangement configuration as to how the heat absorbers are arranged with respect to the flow of the burning-reaction ongoing gas, includes the following two modes. One of those arrangement configurations is that a combustion gas passage is formed so as to allow combustion gas to flow generally linearly therethrough from the burner to the exhaust gas outlet, and moreover the heat absorbers are arranged so as to cross the burning-reaction ongoing gas derived from the burner with gaps present among the heat absorbers to allow the combustion gas to flow therethrough. The other arrangement configuration is that heat absorbers are arrayed in an annular state with gaps present thereamong to allow the combustion gas to flow therethrough, so that the combustion gas derived from the burner flows radially from the inside of the annular heat absorbers toward the heat absorbers, where the heat absorbers are arranged in the burning-reaction ongoing gas derived from the burner. The latter configuration is described in detail in U.S. Pat. No. 6,029,614, the disclosure of which is hereby incorporated by reference.
The third suppression means is what is called exhaust-gas recirculation combustion method. Exhaust gas which has decreased in temperature through endothermic action by the heat absorbers and is then to be emitted to the atmosphere is partly mixed with combustion-use air via an exhaust-gas recirculation passage. The combustion gas temperature is suppressed by a cooling effect of the mixed exhaust gas, by which NOx value is reduced. This third suppression means exerts uniform cooling of combustion gas.
The fourth suppression means is water/steam addition to the burning reaction zone. By this water/steam addition, the burning-reaction ongoing gas is cooled, so that the combustion gas temperature is suppressed and the NOx value is reduced. This fourth suppression means also exerts uniform cooling of the combustion gas. The water/steam addition may be carried out in the exhaust-gas recirculation passage in another embodiment. Besides, in an embodiment in which the burner is provided as a fully-premixing type gas burner and mixed gas of combustion-use air and fuel gas is fed to the burner by a blower, it is possible to perform the steam addition between the burner and the blower. For the water addition, water is added in the form of mist.
Working effects by the combination of the first to fourth suppression means are as follows. Enhancing the functions of the individual suppression means singly would cause drawbacks of the respective suppression means to matter. However, combining the four suppression means makes it possible to achieve super NOx reduction relatively easily without causing the emergence of those drawbacks. In particular, later-described unstable characteristics of the fourth suppression means are alleviated, so that stable NOx reduction can be achieved.
It is noted that the functional enhancement of the first suppression means (premixing high excess-air-ratio combustion) is to increase the excess air ratio. Due to this functional enhancement, there would occur a halt of burning reaction and an unstable combustion of the combustion burner. Also, the functional enhancement of the second suppression means (heat-absorber cooling) is the provision of the heat transfer tubes in contact with the burner or the increasing of the heat-transfer-surface density of the heat absorbers. Due to this functional enhancement, there would occur an increase in pressure loss or an unstable combustion such as oscillating combustion.
Also, the functional enhancement of the third suppression means (exhaust gas recirculation) is to increase the exhaust-gas recirculation quantity. Due to this functional enhancement, there would occur an amplification of the unstable characteristics of the third suppression means. That is, the exhaust gas recirculation has a characteristic that the exhaust-gas flow rate or temperature changes with changes in combustion quantity or changes in load. An increase in the exhaust-gas recirculation quantity would cause these unstable characteristics to be amplified, making it impossible to achieve a stable NOx reduction. Also, due to the functional enhancement of the third suppression means, burning reaction would be suppressed, causing an emission increase of CO and unburned components as well as an increase in thermal loss. Further, increasing the exhaust-gas recirculation quantity would cause the blower load to increase.
Also, the functional enhancement of the fourth suppression means (water/steam addition) is to increase the quantity of water to be added. Due to this functional enhancement, the quantity of condensations would increase with increasing thermal loss, where, particularly in boilers having a feed water preheater for preheating the water fed to the heat absorbers by exhaust gas, there would matter corrosion due to the condensations of the feed water preheater.
According to the embodiment, since the first to fourth suppression means are combined together, the problems that would otherwise emerge upon enhancing the functions of the individual suppression means each singly can be prevented from becoming issues.
Furthermore, in the foregoing embodiment, preferably, an excess-air-ratio control means for controlling the excess air ratio to a specified high excess air ratio is additionally provided. More specifically, an oxygen concentration detection means for detecting the oxygen concentration in exhaust gas is provided, and the rotational speed of the blower for blowing combustion-use air to the burner is controlled so that the oxygen concentration detected by the oxygen concentration detection means becomes a set value corresponding to the specified high excess air ratio. The specified high excess air ratio is determined in the following manner. Given a NOx reduction target value of 10 ppm, an excess air ratio corresponding to the target value is determined under the condition of the excess air ratio versus NOx characteristic of the NOx reduction step, and then the excess air ratio determined in this way or a value higher than the excess air ratio is taken as a specified high excess air ratio. Finally, the specified high excess air ratio corresponds to the NOx reduction target value.
In this connection, the foregoing embodiment includes the following modifications. First, the NOx reduction means for fulfilling the NOx reduction step includes the following five modifications: (1) a mode in which three suppression means of the second suppression means (heat-absorber cooling), the third suppression means (exhaust gas recirculation) and the fourth suppression means (water/steam addition) are combined together excluding the first suppression means (premixing high excess-air-ratio combustion); (2) a mode in which three suppression means of the first suppression means (premixing high excess-air-ratio combustion), the second suppression means (heat-absorber cooling) and the third suppression means (exhaust gas recirculation) are combined together; (3) a mode in which three suppression means of the first suppression means (premixing high excess-air-ratio combustion), the second suppression means (heat-absorber cooling) and the fourth suppression means (water/steam addition) are combined together; (4) a mode in which two suppression means of the second suppression means (heat-absorber cooling) and the third suppression means (exhaust gas recirculation) are combined together; and (5) a mode in which two suppression means of the second suppression means (heat-absorber cooling) and the fourth suppression means (water/steam addition) are combined together.
Although all of these modifications include the second suppression means (heat-absorber cooling), yet this is not limitative. The reason of this is that the present invention, which includes first performing NOx reduction in preference to CO reduction and thereafter performing CO reduction, has no limitation for any particular NOx reduction means, even though some NOx reduction means are preferable. The NOx reduction means in this embodiment is designed for those in which carrying on NOx reduction forward to achieve a NOx reduction target would cause the exhaust CO value to exceed a CO reduction target value. Further, the type and form of the burner to be used for the NOx reduction means is not limited to particular ones, either.
The excess-air-ratio control means includes the following modifications. The foregoing excess-air-ratio control means is designed to control the rotational speed of the blower. Instead, the excess-air-ratio control means may be designed to control the opening of a combustion-use-air flow rate adjusting means such as a damper or a valve provided downstream or upstream of the blower so that the excess air ratio is controlled constant. Further, in another embodiment, it is also possible that an outside-air temperature detection means for detecting outside-air temperature is provided in place of the oxygen concentration detection means, where the blower or the flow rate adjusting means is controlled by this outside-air temperature detection means so that the excess air ratio is controlled constant.
Next, the constitution of the CO reduction step is explained. This CO reduction step is a step in which the value of CO generated and emitted from the foregoing NOx reduction step is reduced to not more than a specified value by the CO reduction means.
The CO reduction step is carried out, preferably, in a zone where the temperature of combustion gas is not more than 900xc2x0 C. It is known that CO, if allowed to stand for a necessary residence time with the combustion gas temperature in a range of 900xc2x0 C. to 1400xc2x0 C., oxidizes into CO2. Unfortunately, an attempt to maintain this temperature would be a constraint on the preferential execution of NOx reduction. However, this constraint can be eliminated by performing the CO reduction in a zone where the combustion gas temperature is not more than 900xc2x0 C. By performing the selection of the CO reduction means, conditions as to thermal resistance are relaxed, so that an easier selection is allowed.
As the CO reduction means, CO oxidation means for oxidizing CO into CO2 is used, preferably, a CO oxidation catalyst is used. This CO oxidation catalyst serves not only for the oxidation of CO but also oxidation of unburned components. The CO oxidation catalyst is a preferable means in terms of its fittability to boilers or other thermal equipment, maintenance performance and cost.
As the CO oxidation catalyst, one which exerts oxidation catalysis at 100xc2x0 C. to 1000xc2x0 C. is selected. The lower-limit 100xc2x0 C. is an activation temperature of the CO oxidation catalyst, i.e., a temperature at which it exerts an effective oxidation catalysis, while the upper-limit 1000xc2x0 C. is a temperature determined in another embodiment thermal resistance of the CO oxidation catalyst. In conclusion, the CO oxidation catalyst is disposed, on the passage along which the combustion gas derived from the burner is distributed, in a zone where the combustion gas temperature is not more than 900xc2x0 C. in terms of the preference of NOx reduction and not less than 100xc2x0 C. in terms of the activation temperature of the CO oxidation catalyst. A specific disposition place of the CO oxidation catalyst is determined in consideration of the boiler body structure of thermal equipment or other factors.
The CO oxidation catalyst is formed so that a base material having air permeability is coated with an oxidation catalyst. The base material, provided by stainless or other metal, ceramic or the like, is subjected to such surface treatment that a wider contact area with exhaust gas can be obtained. The oxidation catalyst is generally provided by platinum, but in another embodiment, may be given by platinum group noble metals, or metal oxides of chromium, manganese, iron, cobalt, nickel or the like.
Next, embodiments of the combustion apparatus for NOx reduction of the present invention are described. The present invention includes the following embodiments of the apparatus corresponding to the foregoing embodiments of the method (1) to (6).
Embodiment (1): A combustion apparatus for NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: NOx reduction means for suppressing combustion gas temperature in such a manner that suppression of NOx generation is preferred to reduction of exhaust CO value, thereby keeping NOx value not more than a specified value; and CO reduction means for reducing exhaust CO value resulting from the NOx reduction means to not more than a specified value.
Embodiment (2): A combustion apparatus for NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: NOx reduction means for suppressing combustion gas temperature so that NOx value in burning-completed gas is reduced to not more than 10 ppm; and CO reduction means for reducing exhaust CO value resulting from the NOx reduction means to not more than a specified value.
Embodiment (3): A combustion apparatus for NOx reduction, comprising: NOx reduction means for fulfilling NOx reduction by a combination of a means for suppressing combustion gas temperature by burning a fully-premixing type burner at a high excess air ratio, a means for suppressing combustion gas temperature by heat absorbers, a means for suppressing combustion gas temperature by recirculating burning-completed gas to a burning reaction zone of combustion gas, and a means for suppressing combustion gas temperature by adding water or steam to the burning reaction zone; and CO reduction means for oxidizing exhaust CO from the NOx reduction means so that the CO value is reduced to not more than a specified value.
Embodiment (4): A combustion apparatus for NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: NOx reduction means for suppressing combustion gas temperature in such a manner that suppression of NOx generation is preferred to reduction of exhaust CO value, thereby keeping NOx value not more than a specified value; and CO reduction means for reducing exhaust CO value resulting from the NOx reduction means to not more than a specified value in a zone where the combustion gas temperature is not more than 900xc2x0 C.
Embodiment (5): A combustion apparatus for NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: NOx reduction means having an excess air ratio versus NOx characteristic that generated NOx value decreases with increasing excess air ratio of the burner, as well as an excess air ratio versus CO characteristic that exhaust CO value increases with increasing excess air ratio; and CO reduction means for reducing exhaust CO value resulting from the NOx reduction means to not more than a specified value, wherein the NOx reduction is performed by burning the burner at an excess air ratio which is determined from NOx reduction target value and the excess air ratio versus NOx characteristic.
Embodiment (6): A combustion apparatus for NOx reduction as defined in any one of the foregoing embodiments (1) to (5), wherein the CO reduction means is a CO oxidation catalyst member.
Furthermore, the embodiments of the apparatus further include the following embodiments (7) to (9).
Embodiment (7): A combustion apparatus for NOx reduction by controlling temperature of combustion gas derived from a burner, comprising: NOx reduction means having an excess air ratio versus NOx characteristic that generated NOx value decreases with increasing excess air ratio of the burner, as well as an excess air ratio versus CO characteristic that exhaust CO value increases with increasing excess air ratio; excess-air-ratio control means for controlling excess air ratio of the burner to a specified high excess air ratio; and CO reduction means for reducing exhaust CO value resulting from the NOx reduction means to not more than a specified value.
Embodiment (8): A combustion apparatus for NOx reduction as defined in the foregoing embodiment (7), wherein the specified excess air ratio is determined from a NOx reduction target value and the excess air ratio versus NOx characteristic.
Embodiment (9): A combustion apparatus for NOx reduction and CO reduction as defined in any one of the foregoing embodiments (7) to (8), wherein the CO reduction means is a CO oxidation catalyst member.
In the foregoing embodiment (3), the NOx reduction means is implemented by a combination of the first suppression means to fourth suppression means. However, in another embodiment, the NOx reduction means may also be implemented according to the five modifications described in the embodiments of the method other than this combination. In the Embodiment (7), the excess-air-ratio control means is similar to that described in the embodiments of the method.
The embodiments (1) to (9) are capable of achieving both NOx reduction and CO reduction at the same time, and the embodiments (7) to (8) are capable of achieving a stable NOx reduction by constant excess-air-ratio control even with outside air temperature varied.
Also, in the foregoing embodiments, the NOx reduction step may include CO reduction by CO reduction means, and the NOx reduction means may include CO reduction means. This CO reduction means is a heat-absorber removal space for CO oxidation (i.e., CO oxidation space) formed by eliminating some of the heat absorbers. As stated before, CO, if allowed to stand for a necessary residence time with the combustion gas temperature in a range of 900xc2x0 C. to 1400xc2x0 C., oxidizes into CO2. The aforementioned space, to which this principle is applied, is a space formed by removing plural ones among the heat absorbers and having such a constitution that the combustion gas temperature falls within the aforementioned temperature range under the continued-combustion condition.